openssl_pkey_new() checks private_key_bits >= 384 before generating any
key. For EC, X25519, ED25519, X448, and ED448 the size is inherent to
the curve or algorithm, so this check doesn't apply and causes failures
when default_bits is missing from openssl.cnf (which is the case in
OpenSSL 3.6's default config).

Skip the minimum-bits check for key types that don't use private_key_bits.

Closes GH-21387.